CHAPTER XXII 



Vestibular mechanisms 



B. E. GERNANDT I Department of Physiology, University of Gothenburg, Gothenburg, Sweden 



CHAPTER CONTENTS 



Anatomy of Labyrinth 

 Crista 

 Macula 

 Innervation of Sensory Cells 

 Mode of Action of Vestibular Apparatus 

 Action of Semicircular Canals 

 Adequate stimulation 

 Inadequate stimulation 

 Caloric stimulation 

 Galvanic stimulation 

 Action of Otolith Organs 

 Labyrinthine Pathways and Reflexes 

 Ascending Fibers 

 Nystagmus 

 Cortical projection 

 Descending Tracts 

 Effects of Labyrinthectomy 



THE INNER EAR Contains an auditory portion, the 

 cochlea, and a nonauditory portion for maintenance 

 of equilibrium and orientation in three-dimensional 

 space. The association of two apparently very differ- 

 ent functions in a single organ may at first seem 

 puzzling, but the explanation for this is found by 

 studying the past history of the ear. In this chapter 

 we are concerned only with the nonacoustic part 

 which we shall refer to as the vestibular apparatus or 

 the labyrinth. This lodges the three semicircular 

 canals and two little membranous sacs, the utricle 

 and the saccule. Their function is to respond to forces 

 of acceleration, retardation and gravitation. In lower 

 vertebrates, in fish and even in amphibians, the sac- 

 cule seems to play an auditory receptor role in the 

 absence of the cochlea. The labyrinthine function is 

 phylogenetically older than that of hearing. 



The labyrinth is by no means the only sensory or- 

 gan concerned with the control of equilibrium. The 



ability of terrestrial man and his close relatives among 

 the vertebrates to maintain equilibrium and orienta- 

 tion with respect to the environment also depends 

 upon the stream of afTerent impulses from other re- 

 ceptor systems. These are a) the eyes (perception of 

 spatial relationships), b) the interoceptors of the mu.s- 

 cles, tendons, joints and viscera and c) the extero- 

 ceptors of the skin (perception of position and move- 

 ment of the f^ody or changes in either function). 



At the beginning of the nineteenth century Floin^ens 

 (34) published the first exact observations on the func- 

 tion of the semicircular canals of pigeons and raliijits. 

 He was able to induce forced movements of the head 

 and body and involuntary rhythmical, conjugate de- 

 viations of the eyes following injury to the canals. 



Since then an immen.se amount of research work 

 has been carried out. The early part of this period 

 was characterized by the struggle to separate the 

 vestibular apparatus from partnership with the coch- 

 lea in the perception of sound and to attribute to it a 

 function quite unrelated to that of hearing. In 1870, 

 Goltz (46) was the first to arrive at the conclusion 

 that the semicircular canals were sense organs con- 

 cerned with maintaining equilibrium. 



The use of cla.ssical histological methods and the 

 observation of equilibrium disturbances resulting 

 from operative interference with the internal ear 

 (section or extirpation) have in the past been the two 

 principal sources of knowledge concerning the struc- 

 ture and function of the laijyrinth, but the answers 

 given to various questions vary considerably in their 

 value. From this it was realized that knowledge of 

 behavior mechanisms in the normal subject was 

 necessary in order to understand the nature and sig- 

 nificance of defects associated with peripheral or cen- 

 tral injuries. Recording of electrical activity from 

 single fibers of the peripheral vestibular nerve or from 

 nuclei within the central nervous system of different 



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